The present invention relates to a stage apparatus which is used in a semiconductor exposure apparatus, inspection apparatus, or the like, and aligns an exposure master disk, an object to be exposed, or an object to be inspected to a predetermined position, and its control method.
In general, as exposure apparatuses used in the manufacture of semiconductor elements, an apparatus called a stepper, and an apparatus called a scanner are known. The stepper projects a pattern image formed on a reticle on a semiconductor wafer on a stage apparatus in a reduced scale via a projection lens while stepping the wafer beneath the projection lens, thus sequentially forming the pattern image by exposure on a plurality of portions on the single wafer. The scanner moves a semiconductor wafer on a wafer stage and a reticle on a reticle stage relative to a projection lens (scan movement), and projects a reticle pattern on the wafer by irradiating it with slit-patterned exposure light during the scan movement. The stepper and scanner are considered as mainstreams of the exposure apparatus in terms of their resolution and superposing precision.
FIG. 21 is a schematic top view of a wafer stage used in such an exposure apparatus.
A wafer 102 to be exposed is mounted on a wafer stage 101 via a wafer chuck (not shown). With reference to an exposure optical system, the position of an exposure optical axis 103 is fixed in FIG. 21. Hence, the wafer stage 101 must move in the X- and Y-directions with respect to the exposure optical axis 103 to expose the entire surface of the wafer. The wafer 102 must also move in the Z- and tilt directions to adjust the imaging focal point, but a detailed description thereof will be omitted herein. The position measurement of the wafer stage 101 in the X- and Y-directions uses a high-resolution laser interferometer to realize high-precision alignment. In order to use the laser interferometer, a reflection mirror 107 for reflecting a laser beam must be provided on the wafer stage 101. However, since this reflection mirror 107 must reflect the laser beam to cover the entire moving range of the wafer stage 101, it requires a length equal to or larger than the moving distance of the wafer stage 101. That is, if Ly represents the stage moving distance in the Y-direction, the length Lx of a reflection mirror for X-measurement requires Ly or more (=Ly+xcex1).
The moving range of the wafer stage 101 need only be slightly larger than the wafer diameter if exposure alone is done. However, in practice, the wafer stage 101 moves not only in an exposure operation but also in other operations.
In order to expose the wafer 102, alignment must be done with high precision with respect to the imaging point. Various alignment methods are available, and a method that irradiates an alignment mark, which has been exposed and transferred onto a wafer in advance, with alignment light to obtain any alignment error amount from light reflected by the mark is prevalently used. In this alignment method, the alignment optical axis center does not often match the exposure optical axis center.
FIG. 22 shows a general wafer stage when an alignment optical axis center 104 does not match the exposure optical axis center 103. Referring to FIG. 22, the alignment optical axis center 104 strikes a wafer at a position a given distance L2 from the exposure optical axis center 103. The wafer stage 101 must move the wafer 102 in the X- and Y-directions to expose the entire surface of the wafer, and must also move it in the X- and Y-directions to irradiate the entire surface of the wafer with the alignment light 104.
For this purpose, the movable range of the wafer stage 101 must be broadened by the displacement between the alignment optical axis 104 and exposure optical axis 103. Consequently, the length of the reflection mirror 107 must be increased by the broadened size of the moving range of the wafer stage.
Referring to FIG. 22, a length Lx2 of the reflection mirror in the X-direction must be increased by a displacement L2 between the alignment optical axis 104 and exposure optical axis 103.
More specifically, in FIG. 22, assume that an X-interferometer optical axis 105 passes through the exposure optical axis center 103, and a Y-interferometer optical axis 106 passes through the exposure optical axis center 103 and alignment optical axis center 104. Let Ly be the stage moving distance in the Y-direction (the distance between the position (solid line) where the wafer stage 101 has moved a maximum distance in a +Y-direction, and the position (broken line) where the stage 101 has moved a maximum distance in a xe2x88x92Y-direction), and L2 be the distance between the exposure optical axis center 103 and alignment optical axis 104. Then, the minimum required length Ly2 of the reflection mirror 107 is given by Ly2=Ly+L2.
Especially, in recent years, the wafer diameter becomes as large as 300 mm to improve productivity. In order to expose the entire surface of the wafer, the wafer stage must have a moving range at least equal to or larger than the wafer diameter. As the wafer alignment position may be different from the exposure position, and wafers must be exchanged, the moving range must be further broadened. Inevitably, the length of the reflection mirror increases.
However, a reflection mirror 107 having such a large length is not preferable since (1) it is hard to prepare a long reflection mirror having a high-precision mirror surface, (2) high cost is required to prepare the mirror surface of such a long reflection mirror, (3) the weight of the reflection mirror itself increases and results in an increase in total weight of the stage, (4) heat produced by a stage driving device increases due to an increase in stage weight, and (5) the characteristics of a control system deteriorate due to a decrease in natural frequency of a mechanical system of the stage.
In order to solve this problem, an arrangement described in Japanese Patent Laid-Open No. 7-253304 has been proposed. This apparatus comprises a laser interferometer distance measuring device, movable mirror, X-Y moving stage, and arithmetic device. The movable mirror has a length shorter than the stage moving distance in the Y-direction, and a plurality of X-interferometers are provided. The spacing between neighboring X-interferometers is shorter than the length of the movable mirror, and the movable mirror is irradiated with measurement light of one of the X-interferometers independently of the current position of the stage and is sometimes irradiated with measurement light from two X-interferometers. Which of the X-interferometers is ready to measure is determined by the arithmetic device based on the value of a Y-interferometer, and a measurement result in the X-direction is obtained. Upon moving the stage in the Y-direction, a new X-interferometer which becomes ready to measure undergoes recovery operation at a predetermined position using the value of the interferometer that has been used in measurement so far. By passing the values in turn, movement over a broad range is measured using the short movable mirror.
According to the arrangement described in Japanese Patent Laid-Open No. 7-253304, size and weight reductions of a position measurement mechanism (reflection mirror) to be mounted on a stage can be achieved. However, since the plurality of interferometers are selectively used, the process throughput lowers, and the measurement precision is not sufficiently high. According to the findings of the present inventors, such shortcomings are caused for the following reasons.
The recovery operation of each interferometer requires a certain time, and cannot fall within one sample time of a stage control system. Hence, even by the recovery operations of the interferometers like in the conventional system, measurement values cannot be continuously passed to a control system.
The X-Y stage must be aligned with high precision, and alignment is done by a control system that uses feedback of the measurement value of the stage position. The stage control system comprises a digital control system based on high-frequency sampling, and uses high-gain feedback. Observation noise of a position measurement system is disturbance to the control system, and if its influence is serious, the closed loop characteristics of the control system cannot be improved.
Oscillation noise inevitably appears in the measurement value of the laser interferometer. Such noise is produced due to electrical noise in the measurement system and mechanical vibration of the reflection mirror and laser interferometers themselves. Hence, upon passing the interferometer value, a value on which this noise is superposed is passed. For example, when the stage is aligned to stop near the switching position of the laser interferometers, chattering including a large number of times of switching of laser interferometers and value passing may occur due to slight vibration of the stage and noise of the switching position measurement system. When the value on which noise is superposed is passed a large number of times, errors accumulate, and the measurement error between the true stage position and the measurement value may become large.
It is, therefore, an object of the present invention to attain size and weight reductions of a position measurement mechanism (e.g., a reflection mirror and linear scale) mounted on a stage even when the moving range of the stage is broadened.
It is another object of the present invention to minimize deterioration of the position measurement precision resulting from switching of position measurement means by switching the position measurement means in units of predetermined work regions (e.g., exposure process regions, alignment regions) so as to reduce the number of times of switching of the position measurement means.
It is still another object of the present invention to provide an arrangement which can attain size and weight reductions of a position measurement mechanism mounted on the stage and can measure the stage position with high precision, even when the moving range of the stage is broadened.
It is still another object of the present invention to prevent the position measurement means from being switched frequently due to vibration of the stage even when the stage is stopped near the switching position of the position measurement means.
It is still another object of the present invention to obviate the need for increasing the length of an object required in position measurement such as a reflection mirror, linear scale, and the like to be mounted on the stage even when the alignment optical axis and exposure optical axis are separated from each other in an exposure apparatus and a device manufacturing method using that apparatus, and to improve the process throughput and position measurement precision.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.